1. Field of the Invention
The present invention relates generally to composite structures that are used in load-bearing applications. More particularly, the present invention is directed to reinforcing the portion of the load-bearing composite structure that is connected to the body or sub-structure that also participates in carrying the load.
2. Description of Related Art
Composite materials typically include fibers and a resin matrix as the two principal components. Composite materials typically have a rather high strength to weight ratio. As a result, composite materials are being used in demanding environments, such as in the field of aerospace where the high strength and relatively light weight of composite parts are of particular importance.
The fibers used in many load-bearing composite structures or elements are unidirectional and continuous. Such unidirectional fibers are particularly useful when the load-bearing structure is relatively long with respect to the width and thickness of the structure. Wing spars, struts, links, frames, intercostals, beams, skins, panels, jet engine blades and vanes are examples of various aircraft structures that can be relatively long and which are designed to carry significant loads.
A major design consideration involves determining how to attach the load-bearing structure or element to the aircraft body or other support structure. In general, the load-bearing structure is bolted or otherwise securely attached to the aircraft body at one or more points along the load-bearing structure. The precise fit needed for a structural joint often requires machining the contact surfaces of the adjoining parts. This presents a problem with respect to load-bearing structures that are made from unidirectional composite materials because such materials are difficult to machine without initiating interlaminar cracks (between the UD plies) that can result in fatigue failure. Another issue is that to be structurally effective, the load-bearing structure often needs to be orthotropic with a majority of fibers oriented in one direction based on the major overall load that is carried by the load-bearing structure. Bearing stresses resulting from bolted joints, however, are better handled by multi-directional fiber orientations, such as quasi-isotropic laminate. Locally reinforcing a load-bearing structure with multi-directional plies to handle bearing stresses is a design challenge and is time consuming and expensive, once in production.
One approach to solving the problem presented by load-bearing structures that are weakened by machining is to simply add more multi-directional plies of unidirectional fiber to the entire structure at the connection location. However, the amount of unidirectional fiber material that must be added in order to raise localized bearing strength can significantly increase the weight and size of the load-bearing structure.
Other reinforcement systems, such as metal brackets and sleeves have been used to increase the strength of the load-bearing structure at the machined connection or attachment points. However these types of reinforcements tend to be bulky, heavy and expensive. In addition, the use of metal brackets and sleeves can create localized stress points at the connection site which can negatively affect the long-term strength of the joint.
Although the reinforcement systems presently being used to strengthen the connection point between unidirectional fiber load-bearing elements and aircraft support structures are adequate, there still is a continuing need to develop improved connection reinforcements which are as lightweight and small as possible while still providing adequate structural strength at the connection site.